表面活性剂对水热-煅烧法制备的钴酸镍微观结构及电化学性能的影响

Surfactants on Microstructure and Electrochemical Properties of Nickel Cobalt Oxide Prepared by Hydrothermal Calcination Method

采用水热-煅烧法,以硝酸钴和硝酸镍作为原料,在泡沫镍上合成钴酸镍。为改善钴酸镍材料的分散性和形貌,使用不同类型的表面活性剂,包括聚乙二醇(PEG)、十六烷基三甲基溴化铵(CTAB)以及十二烷基硫酸钠(SDS),研究了表面活性剂添加对所合成材料微观结构及其作为超级电容器电极材料电化学性能的影响。通过X射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电镜(SEM)以及比表面积(BET)对材料进行表征,确定材料的组分和微观形貌;采用循环伏安法(CV)、恒电流充放电法(GCD)以及交流阻抗法(EIS)对材料的电化学性能进行分析。结果表明,表面活性剂的加入均对材料的微观结构以及电化学性能有一定的提升,且对纳米材料有结构导向作用,其中SDS可以通过与溶液中镍、钴离子形成晶核,控制纳米线生长,添加SDS制备出的钴酸镍具有规则整齐、密集的纳米线结构,且纳米线的团聚减少,比表面积和孔体积最大,这种结构有利于氧化还原反应的进行,从而提高材料的电化学性能。电化学测试结果表明,使用SDS制备的钴酸镍电化学性能最优,与未添加表面活性剂的试样相比,比电容从352 F·g^(-1)提高至1102 F·g^(-1),在20 A·g^(-1)的大电流下循环1000次后电容保持率为84%。

As a new type of energy storage device,super capacitor has the advantages of high charge and discharge power,long cycle life and large working range,and has been widely used in various industries.As the performance of super capacitors depends largely on electrode materials,exploring suitable electrode materials will become a critical step in the development of super capacitors.Among various materials,nickel cobalt oxide has received extensive attention due to its excellent electrical conductivity,high capacity,relatively friendly price and minor environmental pollution.At present,the methods for synthesis of nickel cobalt oxide mainly include hydrothermal method,sol-gel method and electrodeposition method.Among them,hydrothermal method has been commonly used due to its simple operation and relatively uniform product distribution.Surfactants are substances that can change the properties of liquids,therefore the application of surfactant in hydrothermal environments can optimize the morphology of products and improve their electrochemical performance.Here the hydrothermal method was applied to synthesize nickel cobalt oxide,which was formed on nickel foam by using cobalt nitrate and nickel nitrate as raw materials,and different types of additives including polyethylene glycol(PEG),hexadecyl trimethyl ammonium bromide(CTAB),and sodium dodecyl sulfate(SDS)were used as surfactants.The aim was to investigate the effects of surfactants on the phase,morphology and electrochemical properties of the synthesized materials.The preparation of nickel cobalt oxide mainly included pretreatment of foam nickel,synthesis of precursor and calcination of the precursor to obtain the final product.The raw material of cobalt nitrate and nickel nitrate were dissolved in deionized water and urea was added to adjust the pH value of the solution,then the surfactant was added to the above solution.After fully stirred,the solution was transferred to the hydrothermal reactor,in which the nickel foam was placed beforehand and acted as the matrix for the growth of reaction product.After that the hydrothermal reactor was sealed and heated in the oven at 120℃for 6 h to obtain the precursor material.Then the precursor was washed,dried and calcined at 300℃for 2 h to get nickel cobalt oxide.The sample was characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM)to determine the phase,molecular structure and atomic valence,and to observe the micro structure.The electrochemical properties of the sample were analyzed by cyclic voltammetry(CV),galvanostatic charge-discharge(GCD)and electrochemical impedance spectroscopy(EIS).The results showed that the nickel cobalt oxide prepared without surfactant had the morphology of nanowires,but the nanowires distribute unevenly and some of them even aggregate to some extent.Compared with the sample prepared without surfactant,nickel cobalt oxide prepared with PEG(NC-PEG)had relatively slender nanowires,but the distribution of nanowires was relatively sparse;the morphology change of the sample prepared with CTAB(NC-CTAB)was not obvious;the sample prepared with SDS(NC-SDS)showed regular and dense nanowires,with the average length of the nanowires of about 500 nm,the diameter of 30 nm,meanwhile the nanowires grew in the same direction.And the nickel cobalt oxide prepared with SDS also has a larger specific surface area and pore volume.As an anionic surfactant,SDS could provide a template for the growth of nickel cobalt oxide precursor in a hydrothermal environment,ensuring uniform and regular growth of the nanowires.When nickel cobalt oxide was used as electrode material for super capacitor,the regular structure of samples prepared with SDS would conducive to sufficient contact between the electrode material and the electrolyte,therefore promoting the redox reaction and improving the electrochemical performance of the material.Through electrochemical tests,it was found that electrochemical properties of the nickel cobalt oxide samples prepared with surfactant were improved,the improvement degree depending on the type of the surfactant.The specific capacitance of nickel cobalt oxide prepared without surfactant was 356 F·g^(-1),and the capacitance retention rate after 1000 cycles at 20 A·g^(-1) was 80%.And dynamic analysis for this sample showed that diffusion control played a major role in the charging and discharging process.For each sample,the ability for charge storage increased with the following order:nickel cobalt oxide prepared without surfactant(NC),NC-CTAB,NC-PEG and NC-SDS,and the specific capacitance of the above samples were 352,378,564 and 1102 F·g^(-1) respectively.Nickel cobalt oxide prepared with SDS showed bigger improvement in electrochemical performance than with PEG and CTAB.For the sample prepared with SDS,the specific capacitance increased to 1102 F·g^(-1),the capacitance retention rate increased to 84%after 1000 cycles at a high current of 20 A·g^(-1),and the charge transfer resistance was 0.47Ω.The addition of SDS could improve the electrochemical performance of the prepared nickel cobalt oxide,which would provide a way for the preparation of high-performance electrode materials.